London Bridge murderer Khuram Butt was left free to kill despite being a known Islamist fanatic.

The extremist had been under police investigation since 2015 and even appeared on a TV programme called The Jihadis Next Door, where he was part of a group who paid homage to a black Isis-style flag in London’s Regent’s Park.

Butt, named by police yesterday along with accomplice Rashid Redouane,was a known associate of Isis mouthpiece and suspected terror recruiter Anjem Choudary.

Two members of the public had warned police about Butt’s extremism, and he is suspected of links to two notorious Isis murderers.

But he remained free. And on Saturday night, he and Redouane killed seven people and injured 49 in a brutal rampage with a third terrorist who has yet to be identified.

The gang rammed pedestrians with a van on London Bridge then ran amok in Borough Market, hacking and slashing at civilians with knives, before police cut them down in a hail of bullets.

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Mohammed Shafiq, head of the anti-extremism Ramadhan Foundation, said last night: “I am not surprised Butt carried out the attack. There are serious questions for the authorities.

“Many of us in the Muslim community have been demanding action against these extremists, to no avail.”

Shafiq said Butt, 27, had been known for years for his links to Choudary’s vile group al-Muhajiroiun, now banned.

He added: “Choudary, Butt and their group of terrorist sympathisers have been known to authorities, and nothing was done for years.

“I call for an immediate investigation into what the police knew, what was done and why action was not taken against them.”

Choudary has been linked to the recruitment of more than 100 British terror suspects.

The UK’s senior anti-terrorism officer, Scotland Yard assistant commissioner Mark Rowley, brushed aside criticism of how Butt’s case was handled. He said he had seen “nothing yet” to suggest that a “poor decision has been made”.

Rowley confirmed Butt had been known to police and MI5, and that an investigation of him began in 2015.

But he said the authorities never found any evidence that he was planning an attack, so he was moved into the “lower echelons” of the 500 most active terror investigations.

Rowley refused to say when the investgation was downgraded. He added: “It’s not a black and white issue but over a period the operation has fallen to the lower echelons.

“There was no intelligence to suggest this attack was being planned and theinvestigation had been prioritised accordingly.”

Rowley said work was continuing to learn more about the attackers and “whether they were assisted or supported by anyone else”.

Butt was a British citizen whose parents moved here from Pakistan. He lived in Barking, east London.

Redouane, who was a “clean skin” not known to police, also lived in the area. It’s not known how they met each other.

Butt was an Arsenal fan – he wore their shirt as he murdered his victims – and spoke with a London accent. He had worked as a security guard and for London Underground had a job as an office manager for KFC.

He boasted on his CV that he was good at working under pressure – and was a trained first-aider.

But some in Barking who knew him described his descent into extremism.

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A female neighbour said: “He didn’t like it when I was out and about. He didn’t like women. I think he thinks they should be suppressed at home.”

Sky News spoke to a woman who said Butt would stare in a “sinister” way at women out cycling on the estate.

Butt’s own wife wore a full-face veil. They had two young children – a three-year-old and a baby aged about one.

Locals also said Butt had been banned from a mosque for ranting at an imam who urged worshippers to vote in the 2015 general election. He believed democracy was anti-Islamic.

His Whatsapp feed was filled with religious tract.

A former friend said he phoned the anti-terror hotline about Butt in 2015 after seeing how he had become radicalised by YouTube videos of US hate preacher Ahmad Musa Jibril.

And mum-of-three Erica Gasparri said she reported Butt to police after he tried to brainwash her son in a park.

Rachid Redouane has also been identified by police (Image: Metropolitan Police /PA Wire)

Erica told The Guardian she went to the park to confront Butt after her boy came home and told her: “Mummy, I want to be a Muslim.”

She said Butt told her: “I am ready to do whatever I need to in the name of Allah, including killing my own mother.”

Erica added: “I don’t know how fast I went from the park to the police. I gave them four photos of him and they rang Scotland Yard when I was there. They were very concerned.

“Then I heard nothing. That was two years ago. No one came to me. If they had, this could have been prevented.”

Butt featured heavily in Channel 4 documentary The Jihadis Next Door, where he and other extremists were filmed in Regents Park paying homage to a sinister “black flag of Islam”.

Group leader Mohammed Shamsuddin bragged: “The sharia is coming to the UK – this black flag one day is gonna be on 10 Downing Street.